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ApiiI 16, 1962
M. KOVARIK
3,029,331
THERMAL TRANSDUCERS
Filed Jan. 19, 1959
2 Sheets-Sheet 1
I NVE NTOR:
noémir I
April 10, 1962
M. KOVARl K
3,029,331
THERMAL TRANSDUCERS
2 Sheets-Sheet 2
Filed Jan. 19, 1959
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OUTPUT
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5U PPLY
United States Patent 'C?ice
3,029,331
Patented Apr. 10, 1962
2
1
3,029,331
Mojmir Kovarik, Moorabbin, Victoria, Australia, assign
THERMAL TRANSDUCERS
or to Commonwealth Scienti?c and Industrial Research
Organization, East Melbourne, Australia, a corporation
of Australia
Filed Jan. 19, 1959, Ser. No. 787,696
Claims priority, application Australia Ian. 22, 1958
5 Claims. (Cl. 219—20)
A particular embodiment thereof will now be described
in order to exemplify a useful and valuable application
of the principle of the invention. -However, it is not the
purpose of the following description to limit the invention
to the particular application described, since’ the inven
tion is applicable to Widely differing ?elds of industrial
and scienti?c technique.
An accurate temperature controller for maintaining
the temperature of a liquid bath constant basically con
10 sists of a temperature sensing device situated in the liq
uid bath, an electrical heating element also situated in
This invention relates to thermal transducers and has
the liquid bath and a switch operable by the output of the
particular, though by no means exclusive, application to
sensing element to switch the heater on or off as the case
thermal transducers which may be used as stabilizing
requires. The sensing device may be a thermistor whose
means for use in connection with either on-off or propor
tional control systems. In such cases the transducer 15 change in resistanceis converted into an output voltage
which, after ampli?cation, becomes the input signal to
will normally be used as an integrating element in a
the switch. The switch may preferably be an electronic
feed-back loop. Stabilizing means embodying the ther
switch of the type which has operating levels which can
mal transducers of the invention may be applied to many
be accurately and permanently set, and which, when the
control systems to improve the accuracy of the control
by the introduction of an integrating feed-back loop 20 input rises above one level or fallsrbelow another, effects
the breaking or remaking of the electrical supply to the
which, in spite of ‘the long time constants obtainable,
may be cheaply constructed.
-
.
heater
element.
.
‘
In such a temperature control system periodic tempera
In many control systems, particularly those which are
ture ?uctuations of the liquid bath may be observed dur~
.required for accurate control, it is necessary to limit
the oscillations of the controlled quantity. In order to v25 ing which the bath temperature rises above the upper
level set by the electronic switch. This would occur even
reduce the oscillations of the controlled quantity, or
if an ideal thermometer and switch were to be used since
in other words the hunting of' the control system, it is
it arises because the heater supplyv is switched off when
the thermistor reaches the pre-set upper temperature lim—
back loop. The particular type of integrating feed-back 30 it. The temperature of the liquid bath will not fall im
mediately after the supply to the heater is switched oil,
device is determined by the requirement that its charac
since the “thermal inertia” of the heater will cause the
teristics should be adjusted to suit those of the control
temperature to continue the rise for a short time while
system and those of the controlled device with which it
the bath water absorbsthe excess thermal energy from
is designed to operate.
It is known to provide a stabilizing means for control 35 the heater element itself. It has been assumed in the
above discussion that’the bath is substantially the same
systemscomprising a feed-back loop which includes an
frequently necessary to incorporate a stabilizing-means
in the form of an integrating device contained in a feed
electrical integrating network.
In such means the net- ~ '
temperature throughout being stirred continuously, and
that it is well’insulated from its environment.
It is necessary therefore, if the above~mentioned.?uc
ever, the characteristics of many control systems‘ and
their associated controlled devices demand that, for op 40 tuations areto be eliminated, to switch off the power sup
‘ply to the heater element at sometime before the‘ tempera
timum operation, the time constant of the integrating net
ture of the bath reaches its upper level. Preferably, the
work be large. To satisfy this requirement, the elements
power supply to the element is switched off when the
composing the integrating network usually have to be
extra thermal energy of the heater element is just enough
massive and therefore expensive and bulky. ,
Accordingly therefore, an object of this invention-is 45 to raise the bath temperature to the upper level; It
work is generally of the resistance capacitance type. How
to provide a thermal transducer which will serve as an
integrating device and which is ‘applicable to a ‘large
variety of control systems.
may be seen therefore that an additional signal which
takes into account the “thermal inertia” of the heater ele
ment could be added to the output signal of the thermistor
bridge to obtain the desired effect. The thermal trans
The invention therefore comprises a thermal transducer
having: a thermally conductive body, a plurality of spaced 50 ducer of the invention may be used in a feed-back loop
to provide such control since both the heater and the
temperature sensing elements thermally connected to said
transducer are energy storing elements of similar char
body, and at least one electrically operated heater ar
aoteristics. Therefore,v the thermal transducer may be
ranged in or on said body at a speci?c point so as to be
constructed so that its characteristics match those of the
in direct thermal connection with said body at that point,
heater and bath arrangement. If the time constant of
the electrical input to the heater comprising the input
the heater is equal to that of the transducer, then the
signal of the transducer and the output signal or signals
transducer’s output may be madeproportional to the po
of the transducer being dependent upon a combination of
tential overshoot of the controlled temperature. Accord‘
the outputs of the temperature sensing elements. 7
ingly, by suitable adjustment of the proportion in which
More particularly the thermal transducer may have:
a homogeneous thermally conductive body of extended 60 the temperature signal from the thermistor bridge is mixed
with the signal from the transducer, the electronic switch
or elongated form, and an electrically operated heater
arranged in or on each end of the body so as to be in
direct thermal contact therewith, a plurality of tempera
may be made to disconnect the power to the heater at
a time when the energy stored will be just sut?cient to
ture sensing elements thermally connected to said body ~ bring the controlled temperature to the‘ desired value
at points spaced between said heaters, the transducer in 65 Without undesirable over-shoot. Furthermore, it is de
put and output being as above described. It may be
desirable to divide the body of the thermal transducer
into a body-mass or main portion and into a number
of body-elements which carry the heaters and the tempera
sirable that such' a'transducer should be physically inde
pendent of the bath so that it'can be mounted with the
rest of the electronic control system. Finally, the trans
ducer should be constructed with the maximum degree
In such a case 70 of ?exibility, that is, it should be capable of ?nal adjust
ment so that its characteristics are closely matched to
the body-elements will be secured ‘directly to the body
ture sensing elements of the transducer.
mass at the desired points thereon.
those of the temperature control system.
' 3,029,331
3
4
In'the following description reference will be made to
the accompanying drawings in which:
FIGURE 1 illustrates a transducer having two electrical
and sweated to its ends. Each body-element may be con
structed in a manner identical with those shown in FIG
URE 1. The transducer is surrounded‘ completely by
heaters and two thermistors thermally connected by a
common body;
,
insulating material forming'two halves 19 and 20‘ of a
container, while a heat sink‘is provided by the metallic’
.
FIGURE 2 illustrates a transducer which is equivalent
in operation to that of FIGURE 1 and is connected in
a bridge circuit;
'
rod 21 which is connected to a suitable ?xture (not
shown) and which may serve as support for the body-mass
16 and the insulating material =19. Once again, an ad
justment of the mass of the body-mass 16 may be made
7
FIGURE 3 illustrates a transducer having more than
two [temperature sensing elements in the form of resist 10 in a similar manner to that described with respect to ance thermometers; and
FIGURE 1.
_
FIGURE 4 illustrates the transducer of FIGURE 3
The thermal transducer shown in FIGURE 2 has been
viewed in section.
connected in a simple and elementary manner to a bridge
FIGURE 1 illustrates a thermal transducer formed in
circuit. The input which supplies either one of the heat
accordance with the invention and having the desired 15 ers may 'be switched on and off in synchronism with the
properties. With reference to FIGURE 1, the transducer
heater which is immersed in the bath. A suitable voltage
consists basically of a body-mass 5, body-elements 6 and
is taken from the supply terminals and applied to the
7, heater coils 8t and 9 and thermistors, 10 and 11. The
thermistor bridge, one terminal being connected directly
body-elements ‘6 and 7 are detachably connected to the
to the base of each thermistor and the other terminal being
body-mass 5 by screws 12 and 13,'so that they may be 20 connected to the junction of the two resistors R1 and-R2.
replaced byv others of slightly different mass in order to
The other terminal of the resistor R1 is connected to the
obtain a ?nal characteristics adjustment. Both body
center of one thermistor, while the other resistor R2 is
mass 5 and body-elements 6 and'7 may be formed
connected in a similar manner to‘ the other thermistor.
from a suitable metal such as brass. The use ‘of
The output signal is then taken from the junction of the
a low melting point solder in addition or ‘in place of 25 resistor R1 with the center of the thermistor associated
the screws 12 and 13 may be desirable, particularly when _ with the body-element 17 and from the'junction of the
excellent thermal conduction across the interface is essen
resistor R2 and the body-element 18. The output of the
tial. The transducer is supported in a suitably shaped
thermistor bridge is therefore proportional to the di?erence
block of’ insulating material 14 which prevents undesir
in the resistances of the two thermistors and may be used
able heat losses between the transducer and the ?xture 30 as a feedback signal to modify the control of the bath
on which it rests. The top portion of the body»mass 5 is
heater‘. Outputs of various forms may be obtained by
left uncovered so that heat may flow into the atmosphere
which acts as a heat sink connected to the body-mass. "
In order to ensure that the loss to the atmosphere is sub
stantially constant the transducer and insulating block 35
14 are placed inside a container 15 which merely serves
the purpose of a draft excluder.
using one or both heaters having the same or different
signals, by varying the body-mass 16 and by varying the‘
circuitry of the electrical network in which the thermis
tors are connected.
v
'
FIGURES 3 and 4 illustrate yet another embodiment
of the invention, FIGURE 3 giving a general elevational
In operation, the input to the transducer shown in
view while FIGURE 4 giving a sectional view as indi
FIGURE 1 is a’ current ?ow in either, or in special cases
cated. The transducer illustrated in these ?gures is ‘one
both, of the heaters 8 and 9. The thermistors 10 and 11 40 which is particularly suitable for precise and predicable
are connected so as to form part of a bridge circuit, or
other electrical network, so balanced that changes of re
sistance occurring in the thermistors 10‘ and 11 due to
operation and yet is very ?exible in its applications. The
thermally conductive body of this transducer is in the
form of a copper pipe 22 having ?ve brass spools 24,
changes in their temperature caused by the applied heat
25, 26, 27 and 28 wound with suitable wire for use as
?ow constitute, by reason of the circuit, the desired out 45 resistance thermometers soldered onto vit at regular inter
put. The thermally conducting body of the transducer
vals, and also having two identical brass spools 23 and 29
shown in FIGURE 1 is composed of the body-mass 5
and the body-elements 6 and 7 and although not homo
geneous yet it is thermally continuous. The atmospheric
heat sink is provided so that when a series of spaced
input pulses are applied to the transducer its temperature
will not keep rising inde?nitely, as would be the case
if no outlet was provided for the heat.
In order to obtain some degree of adjustment of the
wound with suitable wire'for use as heaters soldered onto.
its ends as shown. The pipe 22 should be homogeneous
in composition and construction and should be uniform
in its outside and inside diameters, since the theoretical
heat ?ow and temperatures are then more easy to calcu
late.
‘
Either by calculation or by calibration the response by
each resistance thermometer to the application of heat
time constant of the transducer shown in FIGURE 1, 55 at either one or both ends of the pipe may be obtained
the body-mass 5 may be replaced by other body-masses
and by combining the response of selected thermometers
of greater or less weight. From a consideration of the
with or without modi?cation, a predicable and accurate
smallest time constant required and the fact that the
transducer response may be obtained having the desired
heater power should be as low as practicable, the body
parameters. It has been found that a large variety of
elements 6 and 7 are made as small as practicable and
60 useful transducer responses are possible and that many
it has been found that heaters 8 and 9 having a maximum
of these would be most dii?cult'to obtain by‘ conven~
dissipation of 1 watt may be used together with small but
tional
methods. Although the mass of the thermally con,
ton-type thermistors 10 and‘ 11 which are sweated onto
ductive body of this transducer cannot be readily varied,
the ends vof the body-elements 6 and ‘7. It has been found
that time constants ranging from one half to sixteen sec 65 since it is not desirable to include thermal connections of
doubtful quality, yet the variety of responses obtainable
onds may be readily obtained from a‘transducer such as
from the transducer by various combinations, of the re
that illustrated in this ?gure. ,It may also be desirable
sistance thermometers is very large. _The'transducer of
to use body~masses of different materials.
FIGURES
3 and 4 may be housed in suitable insulation
Another thermal transducer which is formed in accord
in amanner similar to that illustrated in FIGURE 1 or
ance with the invention is illustrated diagramatically, in '
70 FIGURE 2 or in any other desired manner.
FIGURE 2. Its operation and construction’ is substan
Further modi?cations may be made to'the various em
tially equivalent to that of the'transducer shown‘in and
described with reference to FIGURE 1. The body-mass
16 may be formed frombrass and is rod-like, or, if de
sired, tubular having two body-elements 116 and 117 screwed
bodiments abovedescribed without departing from the
scope of the invention, therefore, I do not wish to limit
myself precisely to the embodiments described. For ex
ample responses of a different form may be obtained by
3,098,831
6
5
for mitigating irregular loss of heat from said body,
whereby the electrical heater input constitutes the input
signal of the transducer, and the output signal or signals
thermally connecting the transducers shown in FIGURES
1, 2, 3 and 4 to a medium having a ?uctuating tempera
ture and which may be the controlled quantity.
It can be seen from the above description that the in
vention has not only provided a solution to the particular
of the transducer are made up of a combination of the
output conditions of the therrnistors.
4. A thermal transducer comprising a homogeneous
problem indicated but has provided a very ?exible device
capable of application to a wide ?eld and a wide variety
thermally conductive body having an elongated shape,
an electrically operated heater at each end of said body
in direct thermal contact therewith, and a plurality of
I claim:
1. A thermal transducer comprising a body having 10 temperature sensing elements thermally connected to said
body at points spaced between said heaters, ,said heaters
good thermal conductivity, a plurality of spaced tempera
being spaced from said temperature sensing elements
ture sensing elements thermally connected to said body,
su?iciently to cause substantially all of the heat from said
and at least one electrically operated heater associated
heaters which ?ows to said temperature sensing elements
with said body at a speci?c point and in direct thermal
connection with said body at that point, said heater being 15 to ?ow through said body, whereby the input to the
transducer constitutes an electrical input signal to one or
spaced from at least one of said temperature sensing ele
more of the heaters and the output of the transducer is
ments sufficiently to cause substantially all of the heat
made up of the conditions of two or more of the tempera
from said heater which flows to said at least one tempera
ture sensing elements.
ture sensing element to ?ow through said body, whereby
the electrical input to the heater constitutes the input sig 20 5. A thermal transducer for use in the conversion of
a time-variable electrical input current into a related elec
nal of the transducer, ‘and the output signal or signals
trical output of predetermined character, said transducer
of the transducer are made up of a combination of the
comprising a thermally conductive body consisting of a
outputs of the temperature sensing elements.
metallic rod or tube of substantially uniform cross-sec
2. A thermal transducer comprising a thermally con
ductive body having a thermally conductive body-mass 25 tion, an electrical heater winding on each end of said rod
in direct thermal contact therewith and for receiving at
and at least two thermally conductive body-elements
least a portion of the electrical input, a plurality of re
thermally connected directly to said body-mass at points
sistance thermometer windings arranged on the rod be
spaced from one another, an electrically operated heater
tween the heater windings at points spaced from one an
in direct thermal connection with each of said body-ele
ments, a temperature sensing element direct thermal con~ 30 other and in direct thermal connection with said body, an
nection with each body-element, and an electrical network ’ output circuit to which at least two of the thermometer
windings are connected so that changes in the resistances
within which said temperature sensing elements are con
of the said windings will produce an electrical output in
nected for responding to changes in the conditions of
said circuit, and said transducer including a thermally
said temperature sensing elements due to temperature
of problems.
'
changes in said thermally conductive body caused by heat 35 insulating casing surrounding said body for substantially
preventing irregular heat losses from said body.
?ow therein, whereby the electrical input to the heater or
heaters constitutes the input signal of the transducer and
References (Iited in the ?le of this patent
the response of said electrical network constitutes or pro
duces the desired output of the transducer.
UNITED STATES PATENTS
3. A thermal transducer comprising a thermally con 40 2,386,903
Lutomirski ___________ __ Oct. 16, 1945
ductive body having a thermally conductive body-mass
2,602,132
Young ________________ __ July 1, 1952
and at least two thermally conductive body-elements in
2,759,089
Ellis ________________ __ Aug. 14, 1956
direct thermal connection to said body-mass at points
2,767,296
Welch _______________ __ Oct. 16, 1956
spaced from one another, an electrically operated heater
2,774,854
Stack ________________ __ Dec. 18, 1956
wound on each body-element, a thermistor secured to
each body-element in direct thermal contact therewith,
and a thermally insulating casing surrounding said body
45
2,841,329
2,887,556
2,898,434
Statsinger _____________ __ July 1, 1958
Djinis et al ___________ __ May 19, 1959
Lemmerman et al _______ __ Aug. 4, 1959